In recent years, in response to a high demand for excellent sound and image quality, various techniques have been developed to improve the electromagnetic characteristics of magnetic recording media.
The proposed techniques include those directed to the intensity of the recorded magnetic field and the depth of recording, wherein two magnetic layers are employed to improve the electromagnetic characteristics of the magnetic recording medium.
Most typical of these techniques, two magnetic layers are employed wherein Hc of the lower layer closest to the support is less than that of the upper layer to facilitate recording in the depths of the magnetic layer, such that the electromagnetic characteristics of magnetic recording media are improved. Recording in the "depths" of the magnetic layer as used herein refers to recording that occurs below the surface of the magnetic layer and at various distance orthogonal to the plane surface of the magnetic layer. Examples of these techniques include those described in JP-B-52-28364 (the term "JP-B" as used herein means an "examined Japanese patent publication"), and JP-A-60-124025, 60-256917, 52-51908, 51-21304, 54-48504, 57-33435, 57-69528, 50-31804, 53-54002, 56-30608, 59-167845, 58-64634, and 61-214223 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
However, even the above described magnetic recording medium comprising two layers does not meet the present demand for excellent electromagnetic characteristics.
Furthermore, with the diversification of systems for recording on magnetic recording media, the adjustment of Hc of the two magnetic layers is not necessarily adequate for all recording systems.
It has been found that although magnetic recording media are required to be adapted to a number of different recording systems such as FM video signal recording, digital video recording, HiFi sound recording, analog sound recording, and digital sound recording, even various magnetic recording media comprising two layers, let alone conventional single-layer magnetic recording media, are still not satisfactory because different recording systems provide different recording magnetic fields and employ different recording depths.
It has also been found that if the difference in Hc between the two magnetic layers is increased to thereby increase the output of recording in the depths of the magnetic layer, the frequency characteristics in the middle range are deteriorated or noise results, thus adversely effecting the practical audio and video qualities.
In order to solve these problems, a magnetic recording medium having a three-layer structure is proposed in JP-A-59-167845. However, it was found that even such a three-layer magnetic recording medium does not provide sufficient electromagnetic characteristics because each of these layers, particularly the outermost layer, is relatively thick. In a short wavelength recording system such as video recording and digital recording in particular, the benefits of such a structure having three or more layers is not realized unless the outermost layer is thinly coated.